The objective of this project is to characterize dose-response relationships, toxicity, and carcinogenic potential of molybdenum trioxide administered by inhalation to rats and mice in 2-year chronic studies. Animals will be exposed for 6 hours per day, 5 days per week for 2 years. A 13-week interim sacrifice including hematology, clinical chemistry, tissue analysis of molybdenum and copper, SMVCE, bone density, and histopathology will be performed. In addition, a 65-week interim sacrifice including hematology, clinical chemistry, bone density, and histopathology will be performed. At the end of two years, all animals will be given gross and microscopic histopathologic evaluations.